The present invention relates to an image recording apparatus, and in particular, to an image recording apparatus suitable for forming a medical image that is used for diagnosis or used for reference, and to a test pattern for evaluating the same.
In the medical field, radiation images have so far been formed on silver halide films to be used for diagnoses made by medical doctors. In recent years, however, technologies for processing digital images have made progress, and there has been increased the number of radiographing apparatuses (modalities) for medical use each being capable of outputting digital image data corresponding to images obtained through radiographing. If images obtained through radiographing can be stored in the form of digital image data, image processing is easy, less space is required for storage, and aging deterioration of an image is hardly caused even when the digital image data are stored for a long time, which is an advantage.
However, even if images obtained through radiographing are stored in a form of a digital signal, a form for visualization is a problem. Namely, when radiation images are offered for diagnoses made by medical doctors, image quality, which is the same as that of a household printer, is not sufficient, and image forming at higher image quality is needed. In an image recording apparatus for recording the medical images as stated above, it is a problem how to make the initial image quality forming capability to be high, and to keep the initial capability for a long time. An image quality control method that is most simple is to make a maintenance contract with a third party. For example, if a user can judge that image quality of outputted images is deteriorated by sudden deterioration of recording accuracy caused by some troubles of the image recording apparatus, the user has only to request the maintenance each time. However, when image quality is lowered gradually day after day, the user sometimes fails to judge the deterioration of image quality, and further, when the user cannot identify the cause of the deterioration of image quality, it sometimes takes a long time to identify the cause before the start of repair.
In this case, in the case of maintenance of the image recording apparatus, it is possible to grasp the deterioration of image quality and to identify the cause therefore to a certain extent, by inputting test pattern image data in the image recording apparatus and by investigating printed out test pattern images. To carry out image quality control on the user side by advancing the aforesaid system, it can be considered that the test pattern image is outputted periodically by the user so that image quality may be checked. In this case, if the test pattern image data are built in the image recording apparatus, it is possible to output a test pattern image in case of need, and it is further possible to evaluate by using the test pattern image even in the case of non-periodical inspection by a person other than the user such as an inspection for shipment and a maintenance.
Incidentally, as a physical evaluation item for image quality, there are mainly given three items of sharpness, granularity and gradation. For forming medical images with high image quality, every one of the aforesaid three evaluation items is required to be included. For improving image quality, each evaluation item needs to be evaluated separately and its cause needs to be grasped. Therefore, the test pattern for evaluating each evaluation item is necessary. Further, with respect to test patterns for evaluating sharpness and granularity, in particular, it is important for image quality evaluation that test patterns for various conditions can be recorded, and therefore, it is preferable that output density, an output image size, a pixel size and a format are set optionally and are recorded on a recording medium.
However, in many cases, a conventional test pattern stored under the state of image data in each of a medical image recording apparatus is a density adjusting test pattern for correcting mainly look-up table (LUT) gradation characteristics. Though the gradation can be evaluated by this test pattern, the test pattern is not suitable for physical evaluation such as granularity and sharpness.
Though the test pattern recommended by the Society of Motion Picture and Television Engineers (SMPTE) (SMPTE pattern) is a test pattern that is also capable of evaluating granularity and sharpness of image quality, on the other hand, all it can do is just evaluation under the specific recording conditions (density and spatial frequency). If LUT of the image recording apparatus is changed in this case, it can also be used as a test pattern whose density is changed. However, it has a problem that it is not suitable for measurement because it is not an exclusive test pattern for obtaining physical evaluation values such as root mean square (RMS) and Weiner spectrum (WS). If the output size is changed, it can be used also as a test pattern for evaluating sharpness wherein a spatial frequency is changed. However, it has a problem that it is not suitable for measurement because it is not an exclusive test pattern for obtaining physical evaluation values such as sine wave response filter (SWRF) and modulation transfer function (MTF).
Further, an SMPTE pattern is a test pattern wherein geometrical patterns are collected so that granularity, sharpness and gradation of image quality may be evaluated separately, and it can be said that a test pattern for combined evaluation prepared by assuming diagnoses has not been available.
Incidentally, the test pattern disclosed in TOKKAIHEI No. 10-157088 is for judging the image lasting quality, and the test pattern disclosed in TOKKAIHEI No. 11-48464 is for detecting inferior jetting of ink in an ink jet recording apparatus and thereby for improving the gradation, and both of them are not a test pattern for conducting evaluation of sharpness and granularity. Furthermore, the invention disclosed in TOKKAI No. 2000-138952 is one related to the test pattern for evaluating the sharpness in a period up to preparation of image data, and it is not one for evaluating sharpness of outputted images.